The primary objectives at Ocean Drilling Program (ODP) Site 1223 were to determine the depositional history, timing, thickness, and hazards associated with the Nuuanu landslide (Stephen, Kasahara, Acton, et al., 2003), which originated on the northeast flank of Koolau volcano and removed 3000–4000 km2 of the island of Oahu (Satake et al., 2002). The resulting debris avalanche swept enormous blocks of rock and associated turbidites north across a wide area of the seafloor. Site 1223 is located in the debris field produced by the Nuuanu and neighboring Wailau landslides (Fig. F1), near the crest of the Hawaiian arch ~260 km northeast of Oahu (22°58.409´N, 155°39.2590´W; water depth = 4235.1 m) (see table T5 in Shipboard Scientific Party, 2003a, and fig. F1 in Shipboard Scientific Party, 2003b). Originally scheduled for continuous coring of the upper 100 m of the sediment column, drilling terminated in Hole 1223A at only 41 meters below seafloor (mbsf) because of time constraints. An unexpected and pronounced increase in sedimentary hardness at 12.7 mbsf resulted in a bit change below Core 200-1223A-2H. Consequently, drilling rates were slower than anticipated, and only 23.54 m of sediments was recovered in six cores.
The sediments recovered in these six cores from Hole 1223A were divided into 14 lithologic units (Shipboard Scientific Party, 2003b, pp. 8–14 and fig. F2). Cores 200-1223A-1H and 2H include lithologic Units 1–4 and consist of eight stacked, unconsolidated volcaniclastic turbidites, each >10 cm thick, as well as an unreported number of turbidites of lesser thickness. Cores 200-1223A-3X to 6X include lithologic Units 5–14. The principal components of this lower section are two thick sandstone beds (lithologic Unit 5 and Subunit 11a), described as crystal vitric tuffs separated and underlain by weakly consolidated, very fine grained volcaniclastic claystones and siltstones.
The age of the Nuuanu landslide has been inferred on the basis of isotopic and paleomagnetic analyses. Normark et al. (1993) proposed an age range of 1.4–2.6 Ma for the landslide based on previous K-Ar determinations of Koolau lavas. Paleomagnetic data from piston cores from the debris field led Kanamatsu and Herrero-Bervera (2002) to conclude that the landslide occurred at 1.77–2.58 Ma. Herrero-Bervera et al. (2002) derived an age range of 1.8–2.1 Ma for this event. Stephens, Kasahara, Acton, et al. (2003) interpreted the paleomagnetic stratigraphy of Hole 1223A to yield a basal age of <2.58 Ma. The present study was undertaken to provide an independent determination of the age of Hole 1223A sediments based on biostratigraphy.